A castle might be habitable, and a château might be a species of stronghold, and thus the mediæval country house might be either one thing or the other, but still the distinction will always be apparent if one will only go deeply enough into the history of any particular structure.
Light and air, which implies frequent windows, have always been desirable in all habitations of man, and only when the château bore the aspects of a fortification were window openings omitted. If it was an island castle, a moat-surrounded château,--as it frequently was in later Renaissance times,--windows and doors existed in profusion; but if it were a feudal fortress, such as one most frequently sees in the Pyrenees, openings at, or near, the ground-level were few and far between. Such windows as existed were mere narrow slits, like loop-holes, and the entrance doorway was really a fortified gate or port, frequently with a portcullis and sometimes with a _pont-levis_.
The origin of the word château (_castrum_, _castellum_, castle) often served arbitrarily to designate a fortified habitation of a seigneur, or a citadel which protected a town. One must know something of their individual histories in order to place them correctly. In the fifteenth and sixteenth centuries, châteaux in France multiplied almost to infinity, and became habitations in fact.
In reality the middle ages saw two classes of great châteaux go up almost side by side, the feudal château of the tenth to the fifteenth centuries, and the frankly residential country houses of the Renaissance period which came after.
For the real, true history of the feudal châteaux of France, one cannot do better than follow the hundred and fifty odd pages which Viollet-le-Duc devoted to the subject in his monumental "_Dictionnaire Raisonée d'Architecture_."
In the Midi, all the way from the Italian to the Spanish frontiers, are found the best examples of the feudal châteaux, mere ruins though they be in many cases. In the extreme north of Normandy, at Les Andelys, Arques and Falaise, at Pierrefonds and Coucy, these military châteaux stand prominent too, but mid-France, in the valley of the Loire, in Touraine especially, is the home of the great Renaissance country house.